It is well known that dielectric matters can be heated by oscillating electric and/or electromagnetic fields. Microwaves, which are generated in a resonant cavity, are the most frequently used kinds of fields. As a rule micro waves are defined as electric/electromagnetic fields oscillating at frequencies exceeding 900 MHz, still better at frequencies exceeding 400 MHz and best of all at frequencies exceeding 300 MHz.
The disadvantage of microwaves is that the heating usually takes place ia a surface zone, where the energy is focused to so called hot spots.
Oscillating electric/electromagnetic fields at frequencies below microwave frequencies are generally generated between two capacitor discs. Dielectric matters are placed in the air space between the discs, It is of frequent occurrence that heating between capacitor discs is disturbed by the formation of sparks.
This can be avoided by coating the capacitor discs with electrically isolating materials having small values on their dielectricity constant and loss factors implying no or little influence on the electric field. Simultaneously the isolating material shall be characterised by a high electric penetration resistance (EP 85319, U.S. Pat. No. 551,273)
It is also known that the addition of dielectric substances influences the dielectric properties of the load, which is to be heated. (U.S. Pat. No. 5,886,081, U.S. Pat. No. 4,790,965)
A drawback tied to dielectric heating is that the field lines are concentrated to relatively defined areas of the load so that these areas become unequally heated, which implies local heat concentration as a consequence. Especially this is valid, if the load has marked edges and/or protrusive parts. Thus there is a serious problem, if the load to be warmed is perishable to any kind of overheating. An example representing a living matter is a concentrate of red blood cells kept in a bag and meant for intravenous transfusion.
Methods designed for a slow warming of blood have been developed partly by utilising convection. (U.S. Pat. No. 4,167,663) and by partly by utilising microwaves, which at low power warm blood in the course of a transfusion (WO 9926690). The power, which is applied to the warming of blood in accordance to WO 9926690, is so low, that the problem tied to an uneven field distribution is negligible. However methods suitable for the fast warming of perishable loads are lacking.
Bags holding red blood cell concentrates to be used for intravenous transfusion are in general stored in refrigerators at 4° C. Two problems exist as a consequence of this temperature as a blood concentrate is viscous and cold.
It takes long time to get it out of a bag. Thus a blood transfusion will be retarded
Before a blood concentrate is transfused intravenously to a patient it has to be warmed, best of all to body temperature. At acute transfusion occasions, efforts are tried to attain rapid warming of bags holding blood concentrate, in general with water-baths. Such a warming process is in spite of all pains time wasting and as a consequence patients do not receive their transfusions in due course of time.
If for example a patient is in a state if chock owing to an accident, a cooling caused by the transfusion entails a danger of life of the patient.
Experiments implying the rapid warming of bags with blood concentrates by applying micro waves as well as traditional capacitive warming have caused local overheating damages, particularly in surface zones and comers. These damages have occurred in form of coagulated blood parts and have had as consequences that patients have died owing to clots of blood.